Five periods of mass extinctions of species have occurred on Earth. We characterize these
bigfive events according to the rate at which species were lost and the proportion of species
lost.

During these events, at least 75 percent of all animal species perished during relatively brief
periods of time.

Generally, we estimate extinction events to last 2 million to 3 million years. The most recent
extinction - the Cretaceous, when the dinosaurs and many other groups disappeared - might have
lasted less than one year.

Humans became a force of nature 75,000 to 100,000 years ago. Since then, especially in the past
several centuries, a large number of species have gone extinct.

A recent paper in the journal
Nature suggests that while the magnitude of human-caused species loss is relatively small,
its rate equals or exceeds some of the
big five extinctions of the past.

The report describes some clever approaches to compare modern trends in population and species
numbers with trends that occurred as far back as a half a billion years ago. A number of possible
factors can confound such comparisons. For example, the fossil record is incomplete and some groups
of species preserve better than others.

The authors of the
Nature paper minimized these confounding factors by limiting comparisons to individual
groups of animals, such as mammals, that are well-studied and well-represented in the fossil
record.

In terms of magnitude, fewer than 2 percent of the 5,570 species of mammals described on Earth
have gone extinct in the last 500 years. This is small relative to the 75 percent loss of such
groups during the
big five extinctions of the past.

The rate of mammal extinction over the past 500 years, however, is a different story. The
average "background rate" of mammal extinctions is two species lost every million years.

The International Union for Conservation of Nature maintains a worldwide list of species
considered "critically endangered," "endangered" and "threatened." If we add all of the mammal
species so classified to the current rate over the next thousand years, then for all intents and
purposes, the sixth great extinction event of Earth is under way.

Efforts are under way to collect sufficient data - current and fossil - to calculate independent
rates of extinction for other major groups of animals, such as lizards and amphibians. Indeed,
within the past two decades, the plight of amphibian species worldwide highlights the need for such
studies.

The authors suggest that a "perfect storm" of conditions might be setting the stage for future
extinctions. These conditions include invasive species that out-compete or simply eat native
species; habitat destruction and fragmentation; disease; and rapid changes associated with global
climate change.

Anthony Barnosky, lead author of the report and a professor of Integrative Biology at the
University of California, Berkeley, noted that it is not too late to avert the sixth great
extinction event.

"So far, only 1 to 2 percent of all species have gone extinct in the groups we can look at
clearly, so by those numbers, it looks like we are not far down the road to extinction. We still
have a lot of Earth's biota to save," Barnosky said.